The objective of this research is to elucidate the mechanisms that regulate the expression of viral genes within a host cell and to determine the intracellular conditions that are necessary for a successful viral infection. The system to be investigated will be bacteriophages T5 and BF23 (a T5-relative) in their host Escherichia coli. Phage mutants that affect particular regulatory processes have been isolated and the phage-specific proteins involved in these processes have been identified. These regulatory proteins will be isolated so that their mechanism of action can be tested in cell-free systems. For example, the shut off of expression of pre-early and early phage genes is thought to involve phage-coded repressors, and tests of repressor activity in an in vitro transcription system will therefore be performed. The turn-on of expression of late phage genes is probably due to a modification of both the host RNA polymerase and the phage DNA. Analysis of the polypeptide chains of the RNA polymerase and of the physico-chemical state of phage DNA that is competent for late transcription will therefore be carried out. The shut-off of expression of host genes is probably due to a phage-coded pre-early deoxyribonuclease that degrades host DNA but not phage DNA. Pre-early phage functions prepare the host cell for a successful infection and useful probe for investigating these pre-early functions is the presence within the host cell of colicinogenic factor Ib, which arrests phage development during the expression of pre-early genes. A study of the pre-early functions involved in this arrest mechanism is therefore proposed. Finally, the phage mutants have provided an excellent base upon which to study the mechanism of replication of phage DNA, and thereby to elucidate the possible role that the specific single-strand nicks of T5 and BF23 play in this process.